When you press the final confirm button in the checkout form, the page of the payment method opens.
There you have two options: 1. To fulfill your card number and proceed the payment or 2. Cancel and go back in the shop.
A confirm mail is sent both times to the customer and to the shop mail.

Is it possible, in case of cancellation, a mail to the customer not to be sent?

The problematic with this is any order that is not confirmed with the order status and the order status being edited from the admin can be anything else than completed which canceled would not be noticeable in this case until a secondary database table could handle the distinction between linguistic settings. Right now, all languages in the order status table on the database are on the same table.

The most generated errors being found on Opencart forum originates from contributed programming. The increased post counters are caused by redundancies of the same solutions that were already provided prior.